Steps towards modeling nutrient export in coastal Californian streams with a Mediterranean climate

Abstract Along the southern California coast, near Santa Barbara, California, we are measuring nutrient export from specific land uses and developing relationships to predict nutrient export at a watershed scale. The area is characterized by a Mediterranean-like climate and short steep catchments producing flashy runoff. Land uses include chaparral, avocado orchards, greenhouse agriculture, open-field nurseries, and residential and commercial development. Sampling sites are located on defined drainages or storm drains that collect runoff from relatively homogeneous areas representing each land use. Stream water samples are taken once a week during the rainy season, every two weeks during the dry season and every 1–4 h during storms; samples are analyzed for nitrate, ammonium, and phosphate. We determine discharge from measurements of stage derived from pressure transducers at all sampling sites. This information is then converted to flux at a high temporal resolution. Several parameters are presented in an initial effort to build relationships for simulating nutrient export based on land use, precipitation and antecedent soil moisture conditions. The objective is to create robust relationships, using parameters in a simple and cost efficient manner, which can be extended to other coastal watersheds with similar land uses and climate. The effort focuses on nitrate and soluble reactive phosphorus (SRP). The relationship of volume-weighted mean nutrient concentration and runoff/rainfall ratios shows promise as a means of predicting nutrient export in flashy streams experiencing a Mediterranean climate.

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